1. Field of the Invention
The invention relates to actuator assemblies for the selective actuation of valves. In particular aspects, the invention relates to improved hydraulic pressure arrangements and fail safe systems for use in such assemblies.
2. Description of the Related Art
Gate valves and other sliding stem-type valves operate by selectively inserting a reciprocable stem into the flow of fluid to stop the flow when desired. Such valves assemblies are often used with subsea wellheads in order to control the flow of oil or gas from the wellhead. Conventional subsea actuator assemblies are used to selectively open and close valves in subsea Christmas trees, manifolds and other assemblies. Examples of such actuator assemblies are described in U.S. Pat. Nos. 4,311,297 and 4,650,151.
Subsea environments create special problems for the operation of such valves. In deep water production systems it is essential that the valves be made insensitive to ambient hydrostatic pressures. In other words, the operation of the valves should not be affected appreciably by the surrounding water pressure. Additionally, it is important that the valves incorporate a fail-safe feature that is intended to maintain the valve in a closed (or, if appropriate, open) position in the event of a loss of control pressure. In conventional designs, mechanical springs are used to bias the stem into the desired closed (or open) configuration. Such designs are often quite effective at shallow depths. However, difficulties arise when they are used at greater depths. Special problems are created by placement of wellheads in deep waters. The greater the water depth, the greater the spring force required to counteract the effects of hydrostatic head pressure on an unbalanced stem area. American Standard API 17D requires that this factor be taken into consideration when specifying the unit depth rating for a valve assembly. Other constraints, particularly those relating to the size and weight of subsea assemblies make it increasingly problematic to simply increase the mechanical spring force for greater depths.
Use of actuator assemblies that are totally sealed, i.e., the stem is sealed from hydraulic pressure, solves the problems of insensitivity and providing an adequate bias force upon the stem. However, the existence of such assemblies is not a complete solution. Completely sealed assemblies create problems when requirements for an independent rotary or linear override mechanisms are specified for the wellhead. In addition, completely sealed assemblies make provision for position indication difficult.
Improvements to the systems of the prior art would be desirable.